1. Field of the Invention
This invention relates to a variable magnification lens system to be used on copying machines, and more particularly to a variable magnification duplicator lens system which is improved in copying performance quality over a wide magnification range.
2. Description of the Prior Art
The duplicator lens systems are generally required to meet the following conditions.
(1) For imaging a flat subject (an original) on a flat surface, the lens system is desired to have, along with uniform image resolution over the entire field area, good image surface characteristics holding the field curvature and astigmatism to the minimum; PA1 (2) The aperture efficiency should be 100% to suppress drops in light amount in peripheral portions of the field area: PA1 (3) The lens system should be compact and have a wide angle of field to meet the demands for compact and small-size copying machines; PA1 (4) The distortion should be ignorably small; PA1 (5) The contrast should have high enough over the entire variable magnification range, especially, in the low frequency range; PA1 (6) The lens system should be bright in terms of light passing power to permit high speed copying operations; and PA1 (7) The system should be composed of a minimum number of lens elements of inexpensive glass material.
Considering that recent copying machine specifications generally include enlarged- and reduced-scale copying functions. In such a magnified duplication, the lens system should satisfy the above-stated requirements.
In order to avoid deteriorations in performance quality in reduced- and enlarged-scale copying operations, it has been known to change the copy scale without varying the distance from an original surface to a sensitive material surface as described, for example, in U.S. Pat. No. 3,865,470 using the so-called zoom lens unit. However, in consideration of the problems which are encountered in case of the zoom lens with a complicate scale varying mechanism, namely, the problems such as increase of the unit size, difficulties of making designs for high performances, increase in cost etc., it has been the general practice to use, instead of a zoom lens unit, a fixed-focus lens unit on ordinary copying machines for office use. In addition, where a fixed focal point lens is used, the so-called symmetrical lens system which has the lens elements arranged symmetrically relative to a stop is employed in most cases to prevent distortion and lateral chromatic aberrations in the position of the natural or equivalent scale which is regarded as a reference scale.
In a symmetrical lens system of this sort, the lens groups on the opposite sides of a stop are inverse to each other in refractive action. Accordingly, as clear from FIG. 13 which explains coma aberrations of a typical symmetrical lens system in equivalent scale position, the coma aberrations of the system as a whole can be corrected by making an adjustment such that the simple coma aberration 11 which is produced by overcorrection of a front lens group on the front side of a stop becomes substantially symmetric relative to the principal light ray P to offset same by the undercorrective coma aberration 12 which is produced by a rear lens group substantially symmetrically relative to the principal light ray P.
In case of a fixed focal point lens unit, its position has to be shifted for a magnified duplication, i.e., toward the original surface (for duplication on an enlarged scale) and toward the sensitive material (for duplication on a reduced scale).
In reduced scale position, the upper light rays above the principal light ray pass through the front lens system close to its optical axis without undergoing strong refractive action of the front lens system, but as they pass through the rear lens system at a position away from its optical axis they undergo its strong converging action, producing undercorrective coma aberration. On the other hand, the lower light rays have a smaller angle of incidence to the front lens system than in the equivalent scale position, and therefore undergo the refraction by the lens system in a weakened degree, producing under-corrective coma aberration as shown in FIG. 14 and exhibiting considerable deteriorations especially in tangential coma aberration.
In enlarged scale position, the upper light rays above the principal light ray P of the rear lens system undergo weakly converging refractive action, changing from undercorrective coma aberration to somewhat overcorrective coma aberration. The lower light rays undergo strong converging action as the angle of incidence to the front lens system is increased as compared with that of the equivalent scale position, producing overcorrective coma aberration and as a whole resulting in overcorrective coma aberration as shown in FIG. 15.
It has been the general practice to prevent the deteriorations in lens performance in enlarged and reduced scale positions of such a fixed-focus lens either by limiting the copy magnification to a narrow range where the performance quality is practically acceptable or by sacrificing the brightness or high light transmitting power of the lens or by restricting the angle of field.